Electricity storage apparatuses storing in a storage battery power purchased from a power system or power generated by solar power generation and the like and supplying the power to load devices in a home when necessary are known. With such an electricity storage apparatus, for example, it is possible to store excess power generated by solar power generation and use the stored power at night. Moreover, if the stored power is used during the day when the power demand is high, it is possible to contribute to alleviating the power demand.
Generally, a power system supplies alternating-current (AC) power and the AC power is supplied to load devices in a home. On the other hand, direct-current (DC) power is used for charging/discharging a storage battery. Therefore, the power conditioner of a storage battery constituting an electricity storage apparatus has to convert DC power to AC power and vice versa. When the power is converted, some power is lost as heat because of, for example, the resistance of the converter. In other words, the output power from the power conditioner is smaller than the input power.
Generally, the power conditioner of a storage battery is designed to have the maximum power conversion efficiency at its rated power. Therefore, when the input/output of a power conditioner is small, the power conversion efficiency is lower and the power loss is increased. In response to this, techniques for suppressing the power conversion loss by charging/discharging a storage battery only when the input/output of the power conditioner is large are proposed (for example, see Patent Literature 1 and 2).
The Patent Literature 1 discloses an apparatus that does not charge/discharge a storage battery when the conversion efficiency representing the ratio of the output power to the input power in power conversion is lower than a threshold. Moreover, the Patent Literature 2 discloses an apparatus discharging a storage battery when the result of subtracting the power generation of a distributed power source from the power consumption of load devices exceeds a threshold and stopping discharge of the storage battery when the subtraction result is equal to or lower than the threshold.